Crush modelling

ABSTRACT

A method of determining the impact resistance of a structure including a crushable material comprises the steps of determining for one or more layers of a finite element of the material during an impact whether the element or layer thereof is to be treated as failing by crushing. If the element or layer is determined to fail by crushing, a load-bearing portion of the structure is defined and the load-bearing portion is treated for the purpose of subsequent calculations as exhibiting an ongoing resistance.

CROSS-REFERENCED APPLICATION

This application is a continuation application of U.S. patentapplication, Ser. No. 12/589,776, filed on Oct. 28, 2009, which is acontinuation application of U.S. patent application, Ser. No.10/931,273, filed on Aug. 31, 2004, both of which are incorporatedherein by reference in their entireties.

BACKGROUND

1. Field of the Disclosure

This invention relates to methods, apparatus and software for modellingthe behaviour of materials which are crushed particularly, but notexclusively, in the context of composite vehicle body parts underimpact.

2. Discussion of the Background Art

It has been recognised for a long time that fiber-reinforced compositematerials, particularly carbon fiber composites have great potential forrevolutionising the auto industry. It is well known that composites arevery light compared to their metal equivalents, even aluminium, and canbe formed into complex shapes that can do the same job as many weldedmetal stampings.

Composites also have the ability to absorb high amounts of energy duringimpacts which make them ideal for automotive, rail or civilapplications. For example, whereas steel can only absorb up to 20kilojoules per kilogram and aluminium approximately 30 kilojoules perkilogram, carbon composites can absorb up to 80 kilojoules per kilogram.

In addition, unlike metallic structures, the crushed material has verylittle residual strength after it has absorbed the energy. Instead, thecomposite material is essentially transformed into small pieces ofdebris and loosely connected fibres after it has been crushed whichmeans that less space is required than in an equivalent metal structure.This is because in a metal structure space must be provided indesignated crumple zones to accommodate the buckled metal.

There is, therefore, a significant incentive to using compositematerials such as carbon fiber composites in mass production vehicles.However, to date they have only been used in very limited applicationssuch as top-end sports cars, motor sport and small, non-critical partsof mass produced cars.

Two significant current disadvantages of composites is that they arerelatively costly and have long manufacturing cycle times. However, asignificant barrier which still remains to their widespread use in theautomotive industry is the ability to be able to model their performancein an impact. This is of course essential to be able to do in order todesign vehicles which are as safe as possible and which will behave in apredictable way in the event of a crash. Although crash performancetesting can be carried out by building prototypes, this is extremelyexpensive and is only practically feasible in the latter stages ofdesign to prove the basic design and calibrate restraint systems. Duringthe earlier stages of design of vehicles made from metal, finite elementanalysis is used to model the behaviour and interaction of the variousmetal parts and to predict their performance in the event of an impact.This means that designs can be proposed, tested and modified usingcomputer modelling with much less reliance on producing and testingexpensive prototypes.

However, this approach does not currently work for crushable materialssuch as composites. The reason for this is that composites absorb energyby a very different mechanism to metallic structures. Metallicstructures absorb energy by plastic folding of the metal, initiated bylocal buckling of the material, which can be characterised by a stressvs. strain curve to good effect. At limit, final failure, which may betearing or brittle fracture, results in the element being unable totransfer load, although its initial volume is essential unchanged.

On the microscopic scale however some materials such as compositesabsorb energy by local crushing of the material, by matrix cracking,fiber buckling and fracture, frictional heating etc. Viewed on a macroscale, the material is essentially crushed or consumed by the impact ona continuous basis, and the volume of the material is reduced as thestructural material is turned to debris.

It is widely recognised in the art that no satisfactory way of modellingthe crush performance of composite materials exists. Existing finiteelement analysis techniques tend to treat elements of composite bytreating the whole element or separate layers thereof as maintainingtheir integrity until the appropriate failure stress value is reached,whereafter the element or layer is simply deleted from the analysis orthe element or layer is deleted from the analysis in a predefinedperiod. In a typical example, this might result in the element beingdeleted with only 5% of its original edge length compressed. Theconventional finite element calculations essentially cannot deal withvery large changes in volume and therefore catastrophically fail theelement where in reality the unimpinged volume of material still had asignificant capacity to absorb energy. This has the effect that theresults of analysis based on such techniques do not correlatesatisfactorily with actual experimental results such that they cannot berelied upon to predict the performance of structures, e.g., automotivesin the event of an impact.

This is clearly a serious drawback of conventional techniques and inpractice means that composite materials are not used or in the few caseswhere they are used, either the structure must be sufficientlyover-engineered to ensure the required minimum level of performance, orextensive prototyping and testing is needed in order to assessperformance, which is of course unduly time consuming and expensive.

There exists a need, therefore, to be able to predict reliably theperformance of composite materials during an impact.

SUMMARY

When viewed from a first aspect the present invention provides a methodof determining the impact resistance of a structure including acrushable material comprising the steps of determining for one or morelayers of a finite element of said material during an impact whethersaid element or layer thereof is to be treated as failing by crushing;and if said element or layer is determined so to fail, defining aload-bearing portion of the structure and treating said load-bearingportion for the purpose of subsequent calculations as exhibiting anongoing resistance.

When viewed from a second aspect the invention provides computersoftware which, when executed on suitable data processing means,determines the impact resistance of a structure including a crushablematerial by determining for one or more layers of a finite element ofsaid material during an impact whether said element or layer thereof isto be treated as failing by crushing and if said element or layer isdetermined so to fail, defining a load-bearing portion of the structureand treating said load-bearing portion for the purpose of subsequentcalculations as exhibiting an ongoing resistance.

When viewed from a further aspect the invention provides a dataprocessing apparatus programmed to determine the impact resistance of astructure including a crushable material, by determining for one or morelayers of a finite element of said material during an impact whethersaid element or layer thereof is to be treated as failing by crushingand if said element or layer is determined so to fail, defining aload-bearing portion of the structure and treating said load-bearingportion for the purpose of subsequent calculations as exhibiting anongoing resistance.

The inventors have recognised that the actual failure mode of crushablematerials during crush can be approximated as giving an ongoingresistance throughout the continuous consumption of the element or layerat the crush front rather than letting the element or layer as a wholesuffer a single rapid failure.

The inventors have realised that the approach in accordance with theinvention gives much more reliable and accurate results in circumstanceswhere a material undergoes crush.

It should be appreciated that in general the resistive force returnedfor the element or layer is not the peak failure stress but is asomewhat lower value which may be calculated from materials theory ordetermined empirically. To give one specific example, for a typical highstrength carbon composite such as T300 in a toughened resin system thecompressive failure stress is of the order of 600 Newtons per squaremillimeter (N/mm2). However, if the material is crushed continually, theresistance to the impactor is of the order of 100 N/mm², i.e.approximately ⅙ of the peak compression strength value.

The invention therefore effectively adds a new failure mode for elementswhich are determined to be those which in reality will undergocrush—i.e. return a resistance force throughout the consumed length ofthe element. The crush front may simply be the forward face of thebarrier impacting the structure although this is not essential and thecrush front could instead be defined elsewhere—e.g. in a fixedrelationship relative to the barrier.

The element or layer which is determined to be failing by crushing couldbe deleted, the ongoing resistance being applied to one or more elementsor layers adjacent the deleted element or layer, and/or another loadbearing portion of the structure. Preferably the load bearing portion isa portion of the element or layer being crushed itself. For example theelement or layer could be resized or redefined (e.g. by splitting), theongoing resistance being distributed across the or each new element orlayer. In both of the foregoing alternatives the barrier is effectivelytreated as being impenetratable (save possibly for an allowance forminimal penetration to avoid computation difficulties at the boundary).The nodes of the element or layer adjacent to the barrier are thereforeprevented from passing through. However both possibilities are to becontrasted with conventional finite element in which analysis rigidbarriers are effectively treated as impenetratable and analysis elementsor layers are simply compressed against the barrier until the failurestress is reached and the element or layer is deleted with no residualeffect.

In presently preferred embodiments of the invention the crush front isallowed to progress across the element or layer so that the spaceoccupied by the element or layer “passes through” the crush front.

The resistance will not in general be a fixed value but rather may be afunction of one or more parameters relating to the element or layer. Ina preferred example the resistance is a function of the thickness of theelement or layer being crushed along the crush front. Additionally oralternatively the resistance is preferably dependent upon the contactarea at the crush front. Preferably for a given element the actual valueof the resistance force is a constant function of the contact area. Inthe simplest case the resistance force could be directly proportional tothe contact area although this is not essential. Additionally oralternatively where the crushable material is a composite material, theresistance may be determined as a function of the lay-up of the layersof the composite, e.g. the order of the layers.

Furthermore in presently preferred embodiments of the invention thecrush resistance is also a function of one or more dynamic parametersrelating to the impact such as the velocity and/or angle with which theimpactor strikes the element or layer in question or the amount ofrotation imparted to it.

The variations with element/layer and/or dynamic parameters may bedetermined by theory, empirically or both. Even if these variations aredetermined theoretically, this does not imply that the correspondingbase value is so determined and vice versa. In practice it is expectedthat at least the variation of crush resistance with angle will beempirically determined since this is very dependent upon the weave of alayer or on each of the layers of a composite material.

Preferably a set of finite elements of the structure is designated asbeing susceptible to crush. The set could comprise all of the elementsin the structure. However the Applicant has realised from empiricalexperience that only a relatively small zone of a composites structurein the immediate vicinity of an impactor will undergo crash. Inpreferred embodiments therefore only a subset of elements is designatedas being susceptible to crush, thereby defining a crush zone. Theseelements are thus allowed to fail through the novel crushing mode of thepresent invention and will therefore require data allowing theirresistance in this failure mode to be calculated. Elements outside thecrush zone will not have the option of failing by crush. However thismeans that it is not necessary to establish data allowing their failureresistance to be determined. Clearly this is beneficial where empiricaldata is used to measure the resistance exhibited during crush since itobviates the need to establish data for areas outside the crush zone.

When it is determined in accordance with the invention that a particularfinite element is in the crush regime, the conventional finite elementanalysis could simply be suspended in favour of the novel crush failuremode set out herein—in other words the conventional finite elementanalysis calculations would simply not be carried out for the particularelement or layer. In at least some preferred embodiments however theconventional finite element calculations are also carried out inparallel so that analysis reverts to these in the event that at any theelement is calculated to have failed due to another, conventionalfailure mode such as shear, tensile or inter-laminar failure at anypoint whilst the element is being crushed. To give one example if thecrush resistance force gives rise to very large bending forces anelement might then fail as a result of tensile stress rather than beingcrushed.

If the force pushing an element through the crush front is notsufficient to overcome the resistive force calculated in accordance withthis invention the element can effectively can move back intoconventional finite element analysis. It should be appreciated howeverthat the element could again pass through the crush front at a laterstage as dictated by the finite element analysis calculations.

Where analysis reverts to the conventional finite element calculationsthe element or layer in question may be deemed thereafter not to becapable of being crushed or to have a degraded crush capability. Forexample the resistance force of the element or layer in question mightbe reduced, for the purposes of any future crush, in proportion to theamount of it which had previously been consumed during the previouscrush phase.

Where, as is preferred, the load bearing portion is a portion of theelement or layer being crushed itself, the load bearing portion could bethe whole element or layer, i.e. the resistance force could conceivablybe applied as a distributed force across the element or layer. Howeverfor consistency with normal finite element analysis it is preferred toapply the force to the individual nodes of the element so that the nodescomprise the load bearing portion. In some embodiments the force isdivided equally between the nodes. In other embodiments the force may bebiased towards one or more of the nodes. The force is preferably dividedbetween nodes that have passed through the crush front and nodes thathave not in proportions according to the amount of the element by areaor penetration distance that has passed through the crush front. To givean example, if 70% of the element had passed through the crush front,70% of the calculated force would be applied to the nodes that had notyet passed through.

The crush resistance which the element or layer will be treated asoffering may, as mentioned above, be determined using materials theory.However, the internal mechanisms at work during crush are often highlycomplex. For example in fiber composite materials they depend on interalia fiber type and sizing, the resin properties, the cure cycle and theweave style. This complexity is one reason why attempts to model crushin the past have failed. However, one of the strengths of the presentinvention is that it is not necessary to calculate or even understandthe internal mechanisms responsible since it has been appreciated thatfor a given set of macroscopic conditions (area of contact withimpactor, velocity, angle of impact etc.) the crush resistance may beapproximated to a single macroscopic value. This value may therefore beobtained empirically by performing tests on small samples (known in theart as “coupons”) of the material in question which thereafter allows itto be modelled in large, complex structures.

In accordance with the invention an element comprising the entirematerial thickness could be modelled together or, where the materialcomprises layers each layer or sub-group of layers could be modelledseparately.

In accordance with the invention, a determination is made for analysiselements or layers as to whether or not they are to be treated asundergoing crush. In embodiments preferred for simplicity thedetermination is made by deciding whether the impactor barrier hasphysically encroached into the space allocated to a given element orvice versa. In terms of the model this amounts to deciding whether anyof the element's nodes have “passed through” the barrier or in otherembodiments a crush front defined in another region of the model space.If failure of the element through a conventional failure mode has notalready taken place, and the supporting structure has not collapsed, itmay then be deduced that the element will undergo crush. In alternativeembodiments an explicit calculation is made of the stress or strain onthe element which is compared with a threshold failure value. Theelement is therefore denoted as being crushed if this threshold value isexceeded. However the determination is made if an element is determinedto be undergoing crush, the treatment in accordance with the inventionis applied.

It will be appreciated that the ability in accordance with the inventionto model the behaviour of materials being crushed does not, as has beenpreviously attempted, require drastically reducing the size of thefinite elements used in the model which would in any event lead to aninordinately large time or computing power requirement. Rather apractical advantage of using an essentially continuous model of thecrush force, as the methods of the present invention may be seen, is toallow element sizes which are the same order of size as would beemployed for an equivalent analysis of a metal structure. This isbecause when an element has been forced into the crush regime, asdetermined in accordance with the invention, and providing the structuresupporting the element in question is capable of withstanding the forcesinvolved, its edge length is no longer compressed against the wall ofthe impactor or other crush front but is effectively permitted to passthrough, subject, of course, to the resistive force on the wall that theprojected edge length, thickness and crush resistance stress etc.dictate.

Although in many cases where the principles of the invention are appliedthe impactor will be a rigid solid object striking the structure, thisis not essential and the impactor could comprise another part or body ofthe structure with sufficient strength and rigidity.

In presently preferred embodiments shell elements are employed althoughalternatively solid or beam or other elements could be used.

In some embodiments it may be preferred, e.g. for reasons ofcomputational efficiency, that the relative velocity of the impactorwall or crush front and the element in question is taken to be constantduring consumption of the element. However, this is not essential andpreferably the relative velocity is adjusted during the passage of thecrush front through the element. Preferably the resistive force ismodified along the length of the element in accordance with apredetermined function of the relative velocity.

The same considerations apply to angle dependence to allow for rotationduring consumption of the element. Indeed in general any parameter onwhich the crush resistance depends may be updated during consumption ofthe element, another example being the thickness, vibration, temperatureetc.

In some preferred embodiments the friction of the crush interface withthe barrier or other crush front may be specified. This is advantageousas it can influence whether a given element is stable enough to undergocrush or whether it fails by another mechanism.

Modelling of the effect of an impact of a structure including acrushable materials in accordance with the invention may be carried outwithout taking damping into account. In some preferred embodimentshowever damping coefficients are specified which could be internal,external or specified globally by the overall finite element analysismodel.

The invention may be applied to any material which can be crushed, i.e.one which disintegrates with little or no residual strength undercertain conditions. Some possible and non-limiting examples includeconcrete, wood, glasses, ceramics, honeycombs and foams. In preferredembodiments of the invention the crushable material comprises acomposite material, more preferably a reinforced-reinforced compositematerial and most preferably a carbon-fiber reinforced resin.

Although the principles of the invention may be widely applied, e.g. aspart of an original analysis model, preferably software implementing theinvention is incorporated into an existing finite element modellingpackage. The type of finite element modelling is preferably non-linearand could be implicit, explicit or another type of analysis mathematics,although explicit non-linear analysis is preferred. In the currentlypreferred embodiment for example, the software is incorporated intoMSC.Dytran (trade mark) explicit non-linear finite element analysissoftware.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the present invention will now be described,by way of example only, with reference to the accompanying drawings inwhich:

FIG. 1 is a schematic flowchart showing the operation of softwareembodying the present invention;

FIG. 2 is a graph showing resistive force against deflection for a testcoupon of a composite material;

FIG. 3 is a graph of deceleration against time for a test cone whichunderwent an impact under controlled conditions,

FIG. 4 shows the sled velocity versus displacement for the experiment ofFIG. 3;

FIG. 5 shows the predicted deceleration profile is shown in FIG. 5; and

FIG. 6 shows the predicted sled velocity.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In a preferred embodiment of the invention, software operating inaccordance with the principles of the invention is incorporated intoMSC.Dytran (trade mark) 2004 finite element analysis package which isavailable from MSC. Software Inc. This known software can be programmedwith failure stress values for composite materials and thus for a givenfinite element of the material can attempt to model the forces on thatelement until the stress it experiences exceeds the failure stresswhereupon the element is deleted. However, in the embodiment of theinvention now being described, this part of the functionality of thesoftware is supplemented. Instead, the process shown in FIG. 1 isfollowed.

In this process, it is first determined, at 2, when there is impactbetween the defined impactor and an element selected as being capable ofcrush of the structure. If there is contact, it is determined, at 4,whether any of the nodes of the element have penetrated the impactor. Ifnone of the nodes has penetrated the impactor, the software moves to thenext main step at 6 in which the element stress is updated. However, ifpenetration is detected, the software moves, at 7, to assess whether theelement connected to the node is already tagged as undergoing crush. Ifit is not the software adds this tag to the node at 8 and then moves onto update the element stress at 6. If the element connected to the nodehad already been tagged as undergoing crush though, a further series ofsubroutines is carried out first at 9. Firstly the contact force is setto zero. Secondly the direction of crush is stored and lastly therelative velocity is stored.

The next main step at 6 is to update the stress on the element. To dothis it is determined, at 10, how many of the nodes of the element havebeen tagged as undergoing crush. If all of the nodes of the element havebeen tagged, the element is taken to have failed and is thereforeremoved from further calculations at 12. If one or more, but not all ofthe nodes is tagged, the software, at 14, projects the crushingdirection in the element co-ordinate system to allow determination ofthe correct direction for material properties to be calculated. It thendetermines the resistance stress of the element from input data(explained in greater detail below with reference to FIG. 2) and thewhole element is tagged as undergoing crush.

Alternatively, if at the assessment step 10 none of the nodes is taggedas undergoing crush, the system simply does nothing, at 16. Whichever ofthe possibilities 12, 14, 16 is encountered, the software then moves to23 where the conventional finite element stress update is undertakenprior to moving on to the third main step of the process in whichcrushing contact is calculated, at 18.

In this stage, a determination is made, at 20, as to whether the elementhas been tagged as undergoing crush. If the element has not been tagged,processing continues within the previous conventional analysis modebefore returning to the beginning of the process shown in FIG. 1.

However, if the element has been tagged, four actions are taken.Firstly, the intersection between the element and the impactor iscalculated. The intersection is calculated to determine the amount ofmaterial being crushed. If a triangle is crushed from a vertex, thematerial being crushed will increase and, as a result, the resistiveforce will increase as the element is consumed through the barrier.Secondly, the crush direction is obtained, thirdly the crush stress isobtained and finally the crush forces are calculated. Thereafter,processing continues within the previous conventional analysis modebefore returning to the beginning of the process shown in FIG. 1.

In order to calculate the predetermined resistance to be fed into themodel described above, a small coupon of the relevant composite materialis subjected to a crush test. In one example, material sections of60.times.30 mm are cut from flat plates and bonded to a 50 mm thickhoneycomb sandwich in order to promote stabilized crush. The outer edgesof each skin presented to the impactor are chamfered at approximately60.degree. to present a sharp edge to minimize the spike in crushresistance exhibited at the start of crushing and thereby minimize therisk of deamination from the honeycomb at the start of crushing wherethe initial failure corresponds to the compressive failure performanceof the element. The honeycomb cells are oriented perpendicular to thedirection of coupon crush and therefore do not absorb significant energybut ensure that the skins do not buckle. A typical plot of resistanceforce exhibited by a coupon versus deflection (i.e. the amount of thecoupon which has been crushed) is shown in FIG. 2. From this it will beseen that throughout most of the range of deflection the force isrelatively constant. By taking a suitable average value for this, theresistance force to be used in the analysis model for a particularmaterial may be determined. Since the coupon has a constantcross-sectional area, there is no variation of the resistance force withcontact area. However in the model the actual value of the resistanceforce is calculated as directly proportional to the contact length.

It will be appreciated that this method of coupon testing provides a lowcost way of determining the stabilized crush properties for a widevariety of lay-ups configurations and angles. Thus typically such testswould be conducted for each of the material constructions used in thestructure to be modelled as crush capable, and optionally each at arange of angles.

In an exemplary application of the embodiment described, arectangular-section cone structure of a T300 carbon fibre compositematerial approximately 85.times.115 mm in section and approximately 455mm long was mounted on a rigid barrier and a rigid sled is propelled ata controlled velocity into the cone. FIG. 3 shows the measureddeceleration of the trolley versus displacement filtered using aButterworth Order4 low pass filter with upper cut-off frequency of 300Hz in this experiment (impact occurring at Displacement=0). From thisthe actual resistance force encountered may be calculated simply fromthe deceleration of the trolley and its mass. FIG. 4 shows the sledvelocity versus displacement for the same experiment.

The cone was modelled using Dytran 2004 software modified as describedabove with reference to FIG. 1. The predicted deceleration profile isshown in FIG. 5 filtered in the same manner as the test results, using aButterworth Order4 low pass filter with upper cut-off frequency of 300Hz. From this it will be seen that the profiles and absolute values ofthe deceleration are similar. FIG. 6 shows the predicted sled velocityand here a remarkable similarity exists between the tested and predictedresults. For example, the prediction of the distance taken to bring thetrolley to a rest was predicted at 327 mm and was measured at 328 mmmeaning that the prediction was accurate to within 1% percent. This ismuch more accurate than could be achieved with the prior art methods.

What is claimed is:
 1. A data processing system which models thebehaviour of a structure during an impact, said structure incorporatinga material which can fail through a crush failure mode, whereby saidmaterial is continuously consumed by disintegrating into debris, thesystem comprising: a computer that: (i) determines for one or morelayers of a finite element of said material during an impact whethersaid element or layer thereof is failing by said crush failure mode;(ii) if said element or layer is determined to be failing by said crushfailure mode, said computer calculates a resistance force and assignssaid resistance force to said element or layer one or more times suchthat a resistance force is assigned to said element or layer for as longas the length of the element or layer is being reduced by said crushfailure mode; and (iii) outputs data from which a predicted impactresistance of said structure can be calculated.
 2. The system as claimedin claim 1 wherein said resistance force is assigned to a portion ofsaid element or layer.
 3. The system as claimed in claim 2, wherein saidcomputer defines a crush front or barrier and allows said element orlayer to pass through said crush front or barrier whilst being crushed.4. The system as claimed in claim 2, wherein said computer applies saidresistance force to individual nodes of the element or layer so thatsaid portion comprises said nodes.
 5. The system as claimed in claim 4,wherein said computer: defines a crush front or barrier and allows saidelement or layer to pass through said crush front or barrier whilstbeing crushed; and divides said resistance force by allocating a firstpercentage of said resistance force to a first set of nodes that havepassed through the crush front or barrier and a second percentage ofsaid resistance force to a second set of nodes that have not passedthrough the crush front or barrier, wherein said first and secondpercentages are calculated either as a function of the area of theelement or layer that has passed through the crush front or barrier oras a function of the distance that said element or layer has passedthrough the crush front or barrier.
 6. The system as claimed in claim 5wherein said first percentage is the percentage of the area of theelement or layer that has passed through the crush front or barrier orthe percentage of the length of the element or layer normal to the crushfront or barrier that has passed through the crush front or barrier. 7.The system as claimed in claim 1, wherein said computer determineswhether the element or layer is failing by said crush failure mode bydetermining whether an impactor barrier has physically encroached into aspace allocated to said element or layer.
 8. The system as claimed inclaim 1, wherein said computer determines whether the element or layeris failing by said crush failure mode by calculating the stress orstrain on the element or layer and comparing said stress or strain witha threshold failure value.
 9. The system as claimed in claim 1, whereinsaid computer defines a crush front or barrier and determines saidresistance force as a function of a thickness of the element or layerbeing crushed along the crush front or barrier.
 10. The system asclaimed in claim 1, wherein said computer defines a crush front orbarrier and determines said resistance force as a function of an area ofcontact at the crush front or barrier.
 11. The system as claimed inclaim 10, wherein for a given element said resistance force has anactual value which is a constant function of the area of contact. 12.The system as claimed in claim 11, wherein said computer defines saidresistance force as being directly proportional to the area of contact.13. The system as claimed in claim 1 wherein said crushable material isa composite material having a plurality of layers, wherein said computerdetermines said resistance force as a function of the lay-up of saidlayers.
 14. The system as claimed in claim 13, wherein said computerdetermines said resistance force as a function of the order of saidlayers in the composite.
 15. The system as claimed in claim 1, whereinsaid computer determines said resistance force as a function of one ormore dynamic parameters relating to the impact.
 16. The system asclaimed in claim 15, wherein said computer determines said resistanceforce as a function of a velocity and/or an angle at which said elementor layer is struck.
 17. The system as claimed in claim 15, wherein saidcomputer determines said resistance force as a function of an amount ofrotation imparted to the element or layer.
 18. The system as claimed inclaim 1, wherein said computer designates a set of finite elements ofthe.
 19. The system as claimed in claim 18 wherein said set is only asubset of all available elements.
 20. The system as claimed in claim 2,wherein said computer carries out finite element calculations on saidelement or layer in addition to assigning said resistance force to saidportion and uses the results calculated by said finite elementcalculations in subsequent analysis instead of said resistance force ifsaid results indicate the element or layer is not failing by said crushfailure mode.
 21. The system as claimed in claim 20, wherein saidcomputer allocates an element a degraded crush capability for futurecrush analysis if the results calculated by said finite element analysisare used.
 22. The system as claimed in claim 1 wherein said finiteelements are shell elements.
 23. The system as claimed in claim 1wherein said finite elements are solid elements.
 24. The system asclaimed in claim 1 wherein said finite elements are beam elements. 25.The system as claimed in claim 1, wherein said computer defines a crushfront or barrier and adjusts a relative velocity between an impactor andsaid element or layer during passage of the crush front or barrierthrough the element.
 26. The system as claimed in claim 25, wherein saidcomputer modifies the resistance force along a length of the element inaccordance with a predetermined function of the relative velocity. 27.The system as claimed in claim 1, wherein said computer defines a crushfront or barrier and adjusts an angle of impact between an impactor andsaid element or layer during passage of the crush front or barrierthrough the element.
 28. The system as claimed in claim 27, wherein saidcomputer modifies the resistance force along a length of the element inaccordance with a predetermined function of the angle of impact.
 29. Thesystem as claimed in claim 1, wherein said computer defines a crushfront or barrier and specifies a friction of the element or layer withthe crush front or barrier.
 30. The system as claimed in claim 1,wherein said computer specifies material damping coefficients.
 31. Thesystem as claimed in claim 1 wherein said crushable material comprises acomposite material.
 32. The system as claimed in claim 31 wherein saidcomposite material is a fiber-reinforced composite material.
 33. Thesystem as claimed in claim 31 wherein said composite material is acarbon-fiber reinforced resin.
 34. A data processing system which modelsthe behaviour of a structure during an impact comprising: a computerthat: (i) determines for one or more layers of a finite element of amaterial during said impact whether said element or layer thereof isfailing by a crush failure mode whereby said material is continuouslyconsumed by disintegrating into debris; (ii) if said element isdetermined to be failing by said crush failure mode pursuant to (i)above, said computer assigns to a portion of said structure an ongoingresistance throughout a consumed length of the element or layer; and(iii) out puts data from which a predicted impact resistance of saidstructure can be calculated.
 35. A data processing system which modelsthe behaviour of a structure during an impact, said structureincorporating a material which can fail through a crush failure modewhereby said material is continuously consumed by disintegrating intodebris, the system comprising: a computer that: (i) determines for oneor more layers of a finite element of said material during said impactwhether said element or layer thereof is failing by said crush failuremode; (ii) if said element or layer is determined to be failing by saidcrush failure mode, said computer assigns to a portion of the structurean ongoing resistance throughout a length of the element or layer, and(iii) outputs data from which the predicted impact resistance of saidstructure can be calculated.